The present invention is directed to an improved electrodeless lamp and method, and particularly to an electrodeless lamp which operates at a high efficiency, and with lower cooling requirement.
The efficiency of a lamp is a measure of the light radiated as compared with the electrical energy which must be provided to the lamp to produce such radiated light. It is a very important measure of lamp performance because an increase in efficiency of only a few percent can lead to substantial savings in operating costs when a lamp is operated over time.
Additionally, electrodeless lamps become very hot during operation, and are typically cooled by directing pressurized air at the bulb. The use of pressurized air has the disadvantage of being noisy, and the cooling requirements of smaller bulbs are proportionately greater than those of larger bulbs. However, if the efficiency of the lamp can be improved, more of the inputted energy is converted to light, while less is converted to heat, and less pressurized cooling air is required, which accordingly generates less noise.
Conventionally, it has been thought that the efficiency (or efficacy) of a particular discharge lamp is dependent on the discharge fill composition and density, and on the power density which is applied to the fill.
In accordance with the present invention, it has been discovered that rotating the bulb at high speed results in increased efficiency. Furthermore, the increase in efficiency which is achieved is substantial. A "substantial" increase in efficiency as defined herein means an increase in efficiency of at least about 5% or greater, wherein "efficiency" is defined as the visible light power radiated by the lamp divided by the microwave or r.f. input power. In accordance with the preferred embodiment of the invention which is disclosed herein, the increase in efficiency which is achieved is about 15%.
Additionally, in accordance with the present invention, the lamp may operate with a substantial reduction in cooling requirement. This allows a less noisy source of pressurized cooling fluid to be used, which is an important advantage. As used herein, a "substantially lower" cooling fluid pressure refers to a cooling fluid pressure which is at least about 20% lower than that which would be necessary in the absence of the increase in efficiency which is provided by the high speed rotation.
By way of reference to the prior art, U.S. Pat. No. 4,485,332 teaches the idea of rotating an electrodeless lamp envelope while impinging at least one stream of pressurized air on it for the purpose of more effectively cooling the lamp. Before the invention in U.S. Pat. No. 4,485,332, pressurized cooling air was impinged on a stationary bulb, with the result that cooling was not as effective, and the lamp had to be operated at lower power densities to prevent overheating.
U.S. Pat. No. 4,954,756 discloses the idea of rotating an electrodeless lamp envelope at high speeds (1500-2500 RPM for envelopes of 0.75 inch to 1.5 inch diameter) to obtain bulb surface temperature equalization and change in the spatial emission properties of the lamp. The patent further teaches that there is an inverse relationship between the required rotation speed and bulb diameter. There is no disclosure that an increase in lamp efficiency is obtained in U.S. Pat. No. 4,954,756.
U.S. patent application Ser. No. 953,056, filed Sep. 30, 1992, discloses the operation of a microwave lamp having a small bulb (internal diameter of about 1/4" or 6.35 mm) in a microwave cavity, while U.S. patent application Ser. No. 976,938, filed Nov. 18, 1992 discloses the operation of a similar bulb which is powered by a coaxial exciter. It is disclosed that when these bulbs are rotated at medium speeds (when bulb diameter is taken into account) of 2000-3000 RPM, isolated discharges and arc attachment effects do not occur. While the avoidance of these effects is important to the success of lamp operation, increased lamp efficiency is not obtained.
In accordance with the broader aspects of the present invention, improved lamp efficiency and a lower cooling requirement are achieved by rotating an electrodeless lamp bulb which contains a light emitting fill at high speed.